Brian, 
The NFPA ratings are "worse" if they are numerically higher (0-4 scale), so
MPK at 2 is a greater health hazard than MEK at 1. There are also ratings
for flammability and chemical reactivity.
See
http://www.nfpa.org/members/technical_code_interpretations/keyword/faqs_for_
nfpa_704_and_the_chem/faqs_for_nfpa_704_and_the_chem.html#whatisit

Tim Reeves
ECD


-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: Friday, July 20, 2001 1:47 AM
Subject: Re: White residue from conformal coating


Hans

I can supply a little chapter and verse on comparisons between MPK and
MEK (figs below in order MPK and MEK

NFPA Health index 2, 1 (whatever that means!)
BP 102, 79.6 °C
TCC FP 7, -9 °C
Vapour density 3, 2.5
Vapour pressure 4.72, 12.71 kPa at 25°C
Heat of vaporisation 9.18, 8.25 cal/g at 25°C
Water solubility 43, 223 g/kg
TLV OSHA (PEL) 200, 200 ppm
TLV NIOSH 150, 200 ppm
STEL OSHA 250, 300 ppm
Oral LD50 1600, 2737 mg/kg

I interpret this as MPK being marginally more toxic than MEK and
somewhat less prone to cause chilling during evaporation, hence slightly
less risk to cause blushing.

Xylene is decidedly more toxic (PEL 100 ppm) but may be marginally
better than MPK for chilling during evaporation, although there would
not be much in it. Toluene would be slightly worse on both counts.

Brian

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